Various sensor systems have been developed for detecting sheet properties “on-line,” such as in a papermaking machine while it is operating. Online control of the moisture content in feedstock such as wood chips and products such as oriented strand board (OSB) and paper board is highly desirable to improve production yield and product quality. Moisture in wood chips is one of the main parameters affecting the production of OSB and biofuels. For example, moisture critically affects the pyrolysis of wood products for the production of biofuels. Online moisture measurements are typically performed using either infrared or microwave absorption or spectroscopy techniques. Infrared techniques are limited to measuring surface moisture and/or low basis weight products due to the low penetration depth of infrared light. They cannot be successfully applied to thick products like wood chips in which moisture stratification is often present. Infrared techniques are also strongly affected by broadband absorbers such as elemental carbon that can be found in various products, in particular recycled paper products.
The most commonly used microwave method of measuring the water content on-line on a paper machine is the resonant-cavity technique. In this technique, the paper travels between the two half cavities of the sensor. The method consists of measuring the change in frequencies of two resonances due to changes in the water content in paper. The two frequencies used include one where the maximum amplitude of the electric field is in the middle of the cavity (i.e. at the paper location) and one where the minimum of the electric field (node) is in the middle of the cavity. The former is called the measure frequency and is most sensitive to the change in dielectric constant in paper (i.e. water content). The measure frequency is approximately 1.8 GHz. The latter is called the reference frequency and is mostly insensitive to changes in the dielectric properties in paper. The reference frequency is approximately 3 GHz. The reference frequency is used to correct for undesirable effects that affect both frequencies such as a slight change in the distance between the two half cavities. This resonant cavity method is quite sensitive to changes in water content but requires a separate temperature measurement in order to be accurate since the resonant-microwave technique is strongly affected by the temperature of the sample being measured. The reason is that the permittivity of water in the microwave range is very temperature sensitive. Thus, microwave sensors generally require an independent temperature measurement being performed as well as a temperature correction algorithm. The temperature corrector applied can be as large as 0.5% moisture per 10° C. change in the sheet temperature. Furthermore, this method provides only a water weight measurement. An independent sensor such as a beta- or gamma-emitter-based sensor is required to measure basis weight. A percent moisture measurement is extracted from the water weight and basis weight measurements.